Non-covalent molecular capsules stand as a central research theme in the field of supramolecular chemistry. Characterized by their dynamic confined architectures assembled through non-covalent interactions, including hydrogen bonding and electrostatic interaction, these capsules hold significant promise for applications in catalysis, medicine, and materials science. Conventionally, the construction of molecular capsules has predominantly depended on pre-embedded complementary motifs within supramolecular hosts, a limitation that restricts their synthetic versatility and adaptability. This study addresses this challenge by presenting the design and synthesis of a water-soluble 2-aminobenzimidazole-functional- ized cavitand 1, which is based on a resorcin[4]arene framework. And we examined the binding of the homologous series of the n-alkanes (n-C₇H₁₆ to n-C₂₀H₄₂) to cavitand 1 using a combination of ¹H NMR spectroscopy (nuclear magnetic resonance spectroscopy), 2D NOESY (two-dimensional Nuclear Overhauser Effect Spectroscopy) and DOSY (diffusion ordered spectroscopy) experiments. Cavitand 1 can form 1∶1 complexes with n-C₇H₁₆ and n-C₁₀H₂₂, and no encapsulation behavior for n-C₁₁H₂₄ because the size of n-C₁₁H₂₄ is too large to form 1∶1 complex and too small to form a dimeric capsule. n-C₁₂H₂₆ to n-C₂₀H₄₂ are good templates for the formation of 1∶2 guest-host capsular complexes. The guest of n-C₁₂H₂₆ fits the space comfortably in an extended conformation and broadened, symmetrical signal patterns were observed in the ¹H NMR spectrum. For n-C₁₅H₃₂ the signals are sharper suggesting a kinetically more stable complex. The conformation of n-C₁₅H₃₂ inside the capsule was determined by 2D NOESY experiments. Cross-peaks between the hydrogen atoms at C(1) and C(3) and at C(1) and C(4), C(2) and C(4) and at C(2) and C(5) were observed. But C(6) is in NOE contact only with C(8). This demonstrates the presence of gauche conformations of four carbon atoms at the ends and an extended chain of carbon atoms in the middle. n-C₇H₁₆⊂1 is a 1∶1 complex and n-C₁₅H₃₂⊂1.1 is a dimeric capsule confirmed by DOSY experiments which reveal diffusion coefficients for n-C₇H₁₆⊂1 (D=2.02×10⁻⁶ cm²•s⁻¹) and n-C₁₅H₃₂⊂1.1 (D=1.54×10⁻⁶ cm²•s⁻¹). Through the Stokes-Einstein relationship, show that the hydrodynamic volume of n-C₁₅H₃₂⊂1.1 is 2.3 times that of n-C₇H₁₆⊂1. Then we investigated host-guest interactions between cavitand 1 and styrylpyridinium SP-Cn (n=1~10) fluorophores in solution using ¹H NMR and UV-Vis absorption spectra. The results indicate that a supramolecular nano-capsule structure was formed with a 2∶1 host-guest stoichiometric ratio (SP-Cn⊂1.1). The electron-poor pyridinium group of SP-Cn is bound within the electron-rich cavity through cation…π interactions. Hydrophobic effects and C—H…π hydrogen bonds drive two cavitand 1 to form a capsule. And the alkyl chain of SP-C10 was compressed to J-shaped conformation by the restricted space. SP-C1⊂1.1 exhibits strong fluorescence in water due to the suppression of aromatic ring rotation within the confined space. Overall, the formation of these molecular capsules is primarily driven by the synergistic action of hydrophobic interactions and C—H…π interactions, providing new insights into the rational design of supramolecular assemblies.

Key words: cavitand, conformation adaptation, hydrophobicity, C—H…π interaction, molecular capsule